We have performed an iron K-edge XAS study of the core dimensions of the Fe 3S4 cluster in P. furiosus three-iron ferredoxin in both oxidized [Fe 3S4] and reduced [Fe 3S4] forms. The mean Fe_S distances of the oxidized and reduced cluster are 2.24 and 2.27 w, respectively, and the mean Fe_Fe distance is 2.67 w for both oxidation states. We conclude that there are only subtle structural differences between oxidized and reduced clusters in P. furiosus three-iron ferredoxin. This conclusion is in contrast with a recent crystallographic analysis of the closely-related cluster of Azotobacter vinelandii ferredoxin I.